DESCRIPTION: (Applicant's Abstract) Tobacco use has been a major cause of diseases and premature death, draining the limited available social and economical resources of our society. Despite considerable progress in understanding fundamental aspects of tobacco/nicotine addiction, knowledge of the detailed cellular, molecular, and biophysical mechanisms that underlie nicotine addiction are lacking. Without understanding the precise location, functional properties, and identity of the receptors that mediate nicotine addiction, there is little hope of developing targeted reagents of potential therapeutic value for smoking cessation. Like other drugs of abuse, nicotine is thought to exert its addictive effects via activation on the mesolimbic dopamine system, commonly viewed as the key neuronal circuitry underlying the brain "reward" system. The addictive effects of nicotine are mediated by activation of neuronal nicotinic acetylcholine receptors (nAChRs) in this system, but the precise mechanism of addiction to nicotine is in dispute. The studies outlined in this proposal will determine the physiological and pharmacological properties of the nAChRs expressed by key dopaminergic neurons within the "reinforcement" circuitry: the ventral tegmental area (VTA). The properties of both the pre- and postsynaptic VTA neuronal nAChRs will be determined by electrophysiological methods. Studies on the properties of both the somatodendritic and axon terminal nAChRs of VTA neurons will focus on an examination of the effects of "smoker's levels" of nicotine on subsequent responses to the endogenous agonist, ACh. The hypothesis that differences in the activation and inactivation properties of pre- vs. postsynaptic VTA nAChRs (and their consequent role in addiction per se) are due to the inclusion, or lack thereof, of the a5 subunit in pre vs. postsynaptic nAChRs will be tested directly by using genetically engineered mice that lack a5 nAChR subunit gene expression. These studies will provide important insights into the nicotine addiction as well as the functioning of the mesolimbic reward system, and supply essential information toward development of therapeutic agents for nicotine addiction.